Field of the Invention
The present invention relates to a focus detection apparatus, an image capturing apparatus having the focus detection apparatus, and a focus detection method.
Description of the Related Art
As a focus detection method performed in an image capturing apparatus, an on-imaging plane phase difference method is used in which focus detection by a phase difference method is performed using focus detection pixels formed in an image sensor. In the focus detection by the on-imaging plane phase difference method, the defocusing direction and the defocus amount can be simultaneously detected by the focus detection pixels formed in the image sensor, and focus control can be performed at a high speed.
U.S. Pat. No. 4,410,804 discloses an image capturing apparatus using a two-dimensional image sensor in which one microlens and a plurality of photoelectric conversion portions are formed in each pixel. The plurality of photoelectric conversion portions are configured to receive light components that have passed through different regions of the exit pupil of an imaging lens via one microlens, thereby dividing the pupil. A correlation amount is calculated from focus detection signals output from pixels (focus detection pixels) each including a plurality of photoelectric conversion portions, and an image shift amount is obtained from the correlation amount, thereby performing focus detection by the phase difference method. Japanese Patent Laid-Open No. 2001-083407 discloses generating an image signal by adding focus detection signals output from a plurality of photoelectric conversion portions for each pixel.
However, in the on-imaging plane phase difference method, the light beam received by the focus detection pixel for acquiring focus detection signals is different from the light beam received by an imaging pixel for acquiring an image. For this reason, the influence of aberrations (spherical aberration, astigmatism, coma aberration, etc.) of the imaging lens on the focus detection signal is different from the influence thereof on an image signal. Therefore, a problem arises in that a difference occurs between a detected in-focus position calculated from the focus detection signal and a best in-focus position for the image signal.